The number of Americans aged 65 or older is expected to reach 70.3 million by the year 2030, representing nearly 20% of the population. Consequently, it is increasingly important to understand the factors underlying age-related changes in physiology and to identify risk factors for adverse events that increase with age. Mobility and walking performance limitations have important implications for physical activities of daily living and quality of life in elderly adult. Elderly adults use more metabolic energy when they walk as compared to young adults and may be the result of impaired walking mechanics or reduced muscle efficiency. The overall goal of this research is to investigate the roles of mechanical energy exchange, mechanical work, and muscle efficiency as determinants of walking metabolic cost in elderly adults. Specifically, this research will (1) determine if inverted pendulum mechanical energy exchange is impaired and mechanical work is greater in elderly adults compared to young adults during walking, and (2) determine if the efficiency of the leg muscles is reduced in elderly adults during cycling and hill walking. The results of this research will offer insight into therapeutic prescription for improving walking performance and mobility in elderly adults.